Ophiolite (ophiolite complex) A sequence of rock types, consisting of deep-sea sediments lying above basaltic pillow lavas, dykes, gabbro, and ultramafic peridotite. Some are the remnants of main oceanic crust, others of crust formed in back-arc basins.

To tread the vastness,

of basin and range,

seeking the truth of that which was,

to walk in deep time,

From range to basin

 

seeking the beginning of an age,

and the last of an archeon,

the shield of the earth,

bones on which we dwell,

the bones of the continent, 2 billion years old

 

peaks melt as they arise,

great heights are youth,

as an Appalachian range dies,

a relic of the past,

greater than our sum,

sutures and overfolds

 

a vast hot spot,

Yellowstone to Raton pass,

as the shield passes west,

the great plume rises still,

a real time view of geology at work

 

 

to the youngest of lands,

the active western shore,

slashing north to close the door,

and compression is the future,

Island arcs in the sky

ancient island arcs break,

upon this vast zone,

tumbling and rising to heights,

only to fall into the sea,

young mountains

 these lands have changed,

and will continue,

through deep time,

as it ever was, to be washed away,

terrane. A fault-bounded area or region which is characterized by a stratigraphy, structural style, and geologic history distinct from those of adjacent areas, and which is not related to those areas by unconformable contacts or facies changes. This usage has recently become frequent in studies of orogenic belts in which terranes can have origins as accretionary wedges, island arcs, and microplates. Initially, ‘displaced’ or ‘allochthonous’ was used to qualify the word ‘terrane’ and to distinguish such terranes from merely ‘suspect terranes’, but all three qualifiers are falling into disuse. 

The arc island hypothosis seeks to answer for the accretion of amongst other places Southern California, and the great disparity of rock and minerals in the landscape. The mountains are very young yet carry rock that is well over a billion years old: how did it get there? The answer could be island arcs created over hotspots much like Japan or Samoa were created and the virgin basalt from the mantle was transported by the Pacific Plate to the North American Plate: easy, right?

As the arcs cross the hemisphere they contact the front edge of the North American Plate. However, the former is moving almost due north while the latter is moving west, hence the friction that is the San Andreas. These movements have occured for nearly 500 million years, resulting in the coast ranges and  small cinder cones, culminating with the Sierra Nevada: a grand, young accretion of many unique terranes, far older than the mountains themselves.